The present invention relates to an impedance detection circuit, an impedance detection device and an impedance detection method which are useful for detecting impedance that an impedance element such as a capacitive sensor has.
As a prior art of an impedance detection circuit, that described in Japanese Laid-Open Patent Application H09-280806 gazette can be cited. FIG. 15 is a circuit diagram that shows this prior art impedance detection circuit. In this detection circuit, a capacitive sensor 51 comprised of electrodes 54 and 55 is connected to an inverting input terminal of an operational amplifier 59 via a signal line 57. And a condenser 60 is connected between an output terminal of this operational amplifier 59 and the said inverting input terminal, and further an AC voltage Vac is applied to a non-inverting input terminal. Also, the said signal line 57 to which the capacitive sensor 51 is connected is wrapped up by a shield line 56 and shielded electrically. And this shield line 56 is connected to the non-inverting input terminal of the operational amplifier 59. Output voltage Vd is obtained from an output terminal of the said operational amplifier 59 via a transformer 61.
An impedance detection circuit according to the present invention comprises at least a voltage follower, a first operational amplifier, a signal line of which one end is connected to an input terminal of the voltage follower and the other end is connected to an impedance element, a shield that shields electrically at least a part of the signal line, a shield voltage applying device that applies shield voltage to this shield, a first impedance that is connected between an output terminal of the first operational amplifier and the signal line, and a signal output terminal that is connected to the output terminal of the first operational amplifier.
Also, the impedance detection circuit according to the present invention comprises at least a first operational amplifier, a second operational amplifier of which both input terminals are in an imaginary short status, a signal line of which one end is connected to one of the input terminals of the second operational amplifier and the other end is connected to an impedance element, a shield that shields electrically at least a part of the signal line, a first impedance that is connected between the output terminal of the first operational amplifier and the signal line, and a signal output terminal that is connected to the output terminal of the first operational amplifier.
Further, the impedance detection device according to the present invention comprises at least the impedance detection circuit, and a terminal that can connect the impedance element to the signal line from outside. In this case, a shield that shields electrically at least a part of a board on which the impedance detection circuit is mounted may further be comprised, and the shield voltage may be applied to the shield.
An impedance detection method according to the present invention includes steps for connecting one of the input terminals of the voltage follower to the signal line and making it into the condition of neither input nor output of current at the input terminal, applying the shield voltage based on the output voltage of the voltage follower to the shield of at least a part of the signal line, and detecting a impedance of a impedance element by the current that flows through the first impedance connected to the signal line.
Also, the impedance detection method according to the present invention includes steps for making both of the input terminals of the second operational amplifier imaginary short, connecting one of the input terminals to the signal line and making it into the condition of neither input nor output of current at the input terminal, connecting the other input terminal to the shield of at least a part of the signal line, and detecting a impedance of a impedance element by amplifying the voltage applied to the impedance element by the first impedance and the first operational amplifier connected to the signal line.